1. Field of the Invention
The present invention relates to a printed circuit board fabrication method and, more particularly, to a fabrication method for forming a printed circuit board having thin film resistors embedded in its circuitry traces.
2. Description of the Related Art
The rapid trend of miniaturization in electronic products requires corresponding reduction in the form factor of electronic components and printed circuit boards. One means of achieving this is to replace discrete passive devices with embedded ones. The so-called embedded design is to make passive devices such as resistors or capacitors in a thin film, and then directly embed these devices in a printed circuit board during the fabrication of the board, enabling the embedded thin film passive devices to be integrated with the circuitry traces and active components on the outer layers of the printed circuit board. Exemplars of this design are seen in U.S. Pat. Nos. 5,079,069; 5,155,655; 5,161,086; 5,261,153; 5,347,258; and 5,466,892.
In embedded passive devices such as embedded resistors, unit thickness resistivity is dependent on the resistive material, and the width of the resistor is dependent on the conductor pattern. Normally, good resistive material has high electrical resistivity and low temperature coefficient of resistivity (TCR), and is easy to process. Suitable materials for embedded resistors comprise two types, namely, thin film metal alloys such as NiP, NiWP, NiCr, and NiCrAlSi, and polymer thick films (PTF) such as inductive composites. In the standard printed circuit board fabrication method, the outer metal layers are etched into circuitry traces, and then passive material is coated on the circuitry traces. If metal alloy thin film material is used, it can be coated on the circuitry traces by electroplating, sputtering, electroless plating, or chemical vapor deposit (CVD). When polymer thick film material is used, it is coated on the circuitry traces using the screen-printing method.
The main drawback of the conventional embedded resistor fabrication process is the large variation in resistance due to device resistivity not matching the circuit impedance. The error range, series inductance, and inductive resistance of the device itself cause the device resistivity not to match the circuit impedance. However, because series inductance and inductive resistance are insignificant, they can be ignored.
The variation in resistance of embedded resistors of metal alloy thin film material is low, but thin film metal alloy embedded resistors have low resistivity. Embedded resistors of polymer thick film material are commonly made of epoxy resin-based material with high resistivity. However, the resistance variation in polymer thick film embedded resistors is high. Therefore, it is desirable to have a printed circuit board fabrication method that eliminates the aforesaid drawbacks. FIG. 1 shows a printed circuit board 1 having carbon resistors 2 embedded therein and electrically connected to the circuitry traces 3 on the top side thereof. In order to minimize the resistance variation, every carbon resistor 2 is made having the same geometric shape. However, when carbon resistors 2 are embedded in the printed circuit board 1, they may have different shapes (see FIGS. 2Axcx9c2D), and an open circuit may form between embedded carbon resistors 2 and the circuitry traces 3. When an open circuit forms between an embedded carbon resistor 2 and the circuitry traces 3, the embedded carbon resistor 2 fails to function.
The present invention has been accomplished under the circumstances in view. It is therefore the main object of the present invention to provide an embedded resistor printed circuit board fabrication method which eliminates the drawbacks of the prior art designs by maintaining the shape of the embedded resistors. In the fabrication method of the present invention, the first step is to prepare a substrate having a conductive layer on top and an insulating layer underneath and then to etch the conductive layer such that it has a number of recesses. Thereafter, resistive material is embedded in the recesses, enabling the first conductive layer to be electrically connected to the lateral edges of the resistive material. Next, a second conductive layer is plated on top of the first conductive layer and the resistive material, and then a layer of etch resist is coated on the second conductive layer with apertures smaller than the recesses above the resistive material. Finally, the first and second conductive layers are etched, leaving a conductive pattern with two layers, both electrically connected to the fully formed resistors. Further, multiple embedded resistor printed circuit boards made according to the aforesaid method may be laminated together, forming a multi-layer printed circuit board. Further, a embedded resistor printed circuit board according to the present invention comprises an insulation layer; a first conductive layer laid on top of the insulation layer and defining a number of recesses; resistive material embedded in the recesses and electrically connected to the first conductive layer; and a second conductive layer laid over the top side of the first conductive layer opposite to the insulation layer, such that the first conductive layer, the resistive material, and the second conductive layer are integrated into an electrical circuit, the two conductive layers having been etched into a conductor pattern.